As I understand it, one of the effects of global warming will be to melt the ice sheets and glaciers pouring an awful lot of cold water into the oceans. Quite apart from sea-level rises this will have the effect of stopping the Atlantic conveyer (Gulf Stream) dead in its tracks. As it is the Atlantic conveyor which keeps our island warmer than it's latitude would otherwise suggest the result will be a cooling of our northern weather patterns and, possibly, the re-creation of the polar ice sheets, more heat reflected back into space and more cooling resulting in the start of the next ice age!
A couple of links to back up what Rick was saying:
Yellowstone Volcano -
http://www.guardian.co.uk/science/blog/2009/jan/06/supervolcano-volcano-yellowstone-earthquake
Mega Tsunami -
http://www.armageddononline.org/dc-tusnami.html
Both have contributions from Prof. Bill (Disaster Man) McGuire, a gentleman I know and greatly respect his opinions.